<p>给你一棵二叉树的根节点 <code>root</code> ，返回其节点值的 <strong>后序遍历 </strong>。</p>

<p>&nbsp;</p>

<p><strong class="example">示例 1：</strong></p>

<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = [1,null,2,3]</span></p>

<p><span class="example-io"><b>输出：</b>[3,2,1]</span></p>

<p><strong>解释：</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/screenshot-2024-08-29-202743.png" style="width: 200px; height: 264px;" /></p>
</div>

<p><strong class="example">示例 2：</strong></p>

<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = [1,2,3,4,5,null,8,null,null,6,7,9]</span></p>

<p><span class="example-io"><b>输出：</b>[4,6,7,5,2,9,8,3,1]</span></p>

<p><strong>解释：</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2024/08/29/tree_2.png" style="width: 350px; height: 286px;" /></p>
</div>

<p><strong class="example">示例 3：</strong></p>

<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = []</span></p>

<p><span class="example-io"><b>输出：</b>[]</span></p>
</div>

<p><strong class="example">示例 4：</strong></p>

<div class="example-block">
<p><span class="example-io"><b>输入：</b>root = [1]</span></p>

<p><span class="example-io"><b>输出：</b>[1]</span></p>
</div>

<p>&nbsp;</p>

<p><strong>提示：</strong></p>

<ul>
	<li>树中节点的数目在范围 <code>[0, 100]</code> 内</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

<p>&nbsp;</p>

<p><strong>进阶：</strong>递归算法很简单，你可以通过迭代算法完成吗？</p>
